Ion source with mosaic ion extraction means

ABSTRACT

An ion source including means for producing a directed stream of ionized gas or plasma wherein a polycellular channeled mosaic element is positioned to transect the path traversed by the plasma so that a potential sheath is established in the plasma. The channel diameter is made less than the Debye length of the ions in the plasma wherefor the ions are accelerated out of the plasma, pass through the channels of the mosaic element and emerge as an ion beam on the other side of the matrix.

United States Patent 1151 3,660,715 Post 1 1 May 2, 1972 541 ION SOURCEWITH MOSAIC ION 3,431,461 3/1969 00110 @1111.

EXTRACTION MEANS 3.552.124 1 1971 Banks et a1...

2,982,858 5 1961 H t' l.. [72] Inventor: Richard F. Post, Walnut Creek,Calif. Dyer e d 3.0l5,032 12/1961 Hoyeretal. ..3l3/63X [73] Assignee:The United States of Amerlca as represented by the US. Atomic EnergyPrimary Examiner-Roy Lake Commission Axxisranl Examiner- Palmer C. DemeoFiled: g 18, 1970 AnorneyRoland A. Anderson [21] Appl. No.: 64,720 [57]ABSTRACT An ion source including means for producing a directed U.S. 3 3tream of ionized gas or lasma wherein a polycellular han. 313/231 neledmosaic element is positioned to transect the path [51] hit. Cl. ..I-I05h1/00 traversed by the plasma so that a potential Sheath is [58] Field ofSearch ..315/1 1 1; 313/63, 231,230 established in the plasma Thechannel diameter is made less [56] Referenc Cited than the Debye lengthof the ions in the plasma wherefor the ions are accelerated out of theplasma, pass through the chan- UNITED STATES PATENTS nels of the mosaicelement and emerge as an ion beam on the other side of the matrix.3,355,615 11/1967 Bihan et a1 ..313/63 2,764,707 9/1956 Crawford et a1..313/63 7 Claims, 2 Drawing Figures Patented May 2, 1972 3,660,715

2 Sheets-Sheet l ILL].

I INVENTOR.

m Richard F Post AT'TORN EY.

Patented May 2, 1972 2 Sheets-Shoot 2 INVENTOR. Richard F. PostATTORNEY.

ION SOURCE WITH MOSAIC ION EXTRACTION MEANS BACKGROUND OF THE INVENTIONThe invention disclosed herein was made under or in the course ofContract No. W-7405-ENG-48 with the United States Atomic EnergyCommission.

lon sources for producing beams of ions used, e.g., in various types ofenergetic particle devices such as particle accelerators, ionbombardment devices, controlled fusion reactors, etc., generally employelectrically biased extractor electrodes for separating ions from aplasma to form the ion beam. Occluded gas ion sources of this type aredescribed in Controlled Thermonuclear Reactions," Glasstone and Lovberg,D. Van Nostrand Company, Inc., 1960, beginning at page 145. In these aswell as in other types of ion sources the ions are extracted fromrelatively large orifices or surfaces of a plasma body of relativelylarge dimension and extraction is obtained by electrostatic attractionwhich often requires the application of relatively high potentials.Difficulties ensue due to electrical discharges between the electrodes,disruption of the plasma and the structures are complicated by thegeometries required.

SUMMARY OF THE INVENTION The invention relates, in general, to theproduction of ion beams for use in various charged particle devices and,more particularly, to an ion source wherein extraction of ions from aplasma body is accomplished by means of a passive mosaic element,defining a plurality of critically dimensioned cellular channels,disposed in proximity to the plasma body.

An ion source suitable for practice of the invention includes a meansfor producing a defined body of plasma generally in accord with variousmeans well-known in the art. Such means usually include a means forsupplying a suitable gas together with means for ionizing and heatingthe gas to provide the plasma body. One such suitable plasma producingmeans is that utilized in the above-referenced occluded gas ion source,i.e., an arrangement of hydrided metallic elements across which anelectrical discharge is applied to release and ionize hydrogen gas toform the plasma. Other suitable devices may produce a plasma byinstituting a continuous or pulsed electrical discharge in a suitablegas, for example, between electrodes positioned therein, by means ofradio frequency energy applied thereto, or the like. Usually .the plasmawill be produced in a chamber defined by enclosure wall surfaces.Moreover, the plasma body may be constrained within a magnetic field asdiscussed more fully hereinafter.

For extraction of ions from such a plasma body, in accordance with theinvention, a porous mosaic element formed of either insulating orconductive material is placed in proximity to the plasma, for example,as a portion of the enclosure wall, at a location from which extractionof the ions is desired. For effective extraction to occur, certaincritical conditions must be satisfied. The mosaic element should definea large plurality of reasonably uniform diameter capillary channelspassing in substantially straight line relation therethrough. Variouspolycellular or honeycomb type materials are satisfactorily providedthat the channel diameter is smaller than the Debye length of theplasma. Furthermore, the plasma body dimensions must be at least oneorder of magnitude and preferably at least several orders of magnitudegreater in extent that the Debye length thereof.

With these conditions satisfied a potential sheath is produced in theplasma as if the porous mosaic element was a solid wall similar to theremainder of the enclosure. The potential sheath is of a polarity andthe potential thereacross is of a magnitude sufficient to acceleratepositively charged ions out of the plasma body. The ions having avelocity vector sufficiently parallel to the axis of the mosaic channelspass therethrough to emerge on the opposite side of the mosaic elementas an ion current or ion beam.

If the first condition noted above is not satisfied, i.e., if thechannel diameter exceeds the Debye length the potential sheath necessaryto effect extraction of the ions will not form as if the capillarychannels were not present. If the second condition is not satisfied atrue and appropriate plasma body will not be present.

Accordingly it is an object of the invention to provide an ion source inwhich extraction of ions from a plasma body is effected without the useof electrically energized extraction electrodes.

Another object of the invention is to provide an ion source includingmeans for producing a confined plasma body wherein extraction of ionsfrom the plasma body is effected by means of a passive channeled mosaicelement disposed in proximity to the plasma.

Still another object of the invention is to provide an ion sourceincluding means for producing a plasma body wherein a mosaic element isdisposed with one surface thereof in proximity to the plasma body, saidmosaic element having a plurality of straight line capillary channelsformed therein, which channels have a diameter less than the Debyelength of the plasma so that a potential sheath is developed in theplasma effective to accelerate and extract ions therefrom to passthrough said channels to provide an ion beam emerging at a secondsurface of said element.

Other objects and advantageous features of the invention will beapparent in the following description and accompanying drawing of which:

FIG. 1 is a longitudinal cross-sectional view of an ion source,utilizing a channeled mosaic element for extracting an ion beam inaccordance with the invention; and

FIG. 2 is a schematic illustration of a second embodiment of the ionsource of the invention.

DETAILED DESCRIPTION OF THE INVENTION A plasma body such as thatproduced in the ion source of the invention is an electrically neutralcollection of charged particles, i.e., a collective mass of electronsand positive ions in which the ions and electrons generally have aMaxwellian distribution in velocity space but in which the ion andelectron temperature may be difierent. The ion temperature is generallyless than or equal to the electron temperature, usually less. With ahydrogen plasma the random ion current is at most about one-fortieth ofthe random electron current [(mass electron/mass proton)" 1/42.]

However, if such a plasma is confined within a container or enclosurewith solid walls, the electron and ion currents reaching the boundarywall must be equal otherwise an absurdly high positive charge would becontinuously built up in the plasma. The phenomenon that equalizes theion and electron currents escaping from the plasma is termed the plasmasheath. When the plasma is formed in the container more electrons thanions are initially lost resulting in the accummulation of a positivecharge in the plasma which slows escape of electrons and acceleratesloss of positive ions until the escape currents are equal. The maineffect is to reduce the electron current.

The magnitude of the potential difference between the plasma body andthe wall, the sheath potential drop is of the order of a few timeskTe/e. This potential drop occurs over a characteristic distance whichis of the order of the Debye length which is defined as follows:

A (kTe/41rne (cgs units) wherein Te is the electron temperature in K, neis the electron density, k is the Boltzman Constant and e is theelectron charge. Substituting numerical values:

The Debye leng tli is a measure of the distance into the plasma whereinvariations in the potential can occur.

Now it may be noted that, if a section of wall having openings thereinextending normal to the plasma surface is substituted for a solidsection of the plasma container wall, a potential sheath forms in theplasma in proximity thereto as long as the openings or pores in the wallsection are smaller than the Debye length of the plasma. ions areaccelerated out of the plasma by such potential and impinge upon thewall and enter the openings therein. Provided that the openings are inthe form of straight channels extending through the wall section, ionshaving a sufficiently large velocity vector parallel to the axis of saidchannels will pass therethrough and appear as an ion beam on theopposite side thereof. With a plasma having a density of the order ofparticles/cc, the requisite channel diameter is of the order of IOmicrons. With lower density plasmas, the Debye length is larger, and alarger channel size can be used. Generally speaking, channel sizes ofless than about 50 microns can be used and the porous wall section canbe conveniently provided as a mosaic element.

The transmitted particle beam comprises accelerated ions and deceleratedelectrons. A important advantage to be gained by use of a porous wallsection, i.e., the mosaic element for extracting the ions is evidencedby the following consideration. Any device that is used to a acceleratecharged particles incurs the limitations imposed by the Child-LangmuirLaw (c.f. Electric Phenomena in Gases" Papoular, R. lliffe Books, Ltd.,London 1965, p. 102) which gives the maximum current density, j,,,, thatcan be accelerated by a potential difference, V, between two grids adistance, d, apart, as follows:

where m mass of the ion.

By using the mosaic element to create a sheath potential foraccelerating ions out of the plasma body, the distance over whichacceleration occurs is of the order of the Debye length which is a verysmall distance, of the order of up to a few tens of microns in arelatively dense plasma, i.e., with a density of the order of 10"particles/cc. With denser plasmas the distance, d, is even shorter.Accordingly, an ion beam of high current density can be produced evenwith a sheath potential of a few volts using such a mosaic element. Thelimit would be the ion current reaching the walls of the container andthe transmissivity of the matrix element. A preponderance of ions passthrough the element. The energy thereof will depend on the sheathpotential as well as the initial kinetic energy of the particle in theplasma. Various means are known in the art for increasing such a sheathpotential and accordingly the ion energy of the emergent ion beam mayrange upwardly from a few electron volts determined by the kineticenergy (temperature) of the ions in the plasma added to the energysupplied by the sheath potential. The ion current emerging from themosaic element may reach densities at which charged particle repulsioncauses the beam to blow up" in which case low energy electrons may beintroduced for space charge neutralization to offset the repulsiveforces. Current densities of the order of about 0.1 to above about 1.0amps sq cm may easily be obtained.

DESCRIPTION OF AN EMBODIMENT An ion source utilizing an occluded gassource means for producing a plasma from which ions are extracted toform an ion beam in accordance with the teachings of the invention isillustrated in the FIG. 1 of the drawing. As illustrated therein the ionsource is constructed within a generally cylindrical housing 11 definedan evacuable chamber 12. Housing 11 may be constructed with acylindrical section 13 of stainless steel, glass, ceramic or othertubing material suitable for vacuum service and with a hermeticallysealed cover plate 14 attached, e.g., by flanges or the like to one endof section 13. The other end of cylindrical section 13 may be providedwith an annular base plate 16, i.e., centrally apertured plate 16, whichcan serve as means for attaching the ion source in vacuum tight relationto a part of a vacuum housing 17 of a device, e.g., particleaccelerator, fusion reactor, etc., in which the ion beam is to beemployed. While such source mounting plate might be attached directly tohousing 17, in order to facilitate making certain measurements, anelectrically insulated mounting may be used. For this purpose, anannular member 18, of insulating material, e.g., polymerized acrylicresin or the like, is inter posed between plate 16 and housing 17 andthe plate 16 is attached thereto by means of flange bolts 19 insulatedfrom plate 16 by means of a washer 21 and an insulating sleeve 22.

The occluded gas plasma source 23 used herein, is generally constructedwith an assembly comprising a plurality of annular metallic washers 24interleaved with and separated by means of thin insulating annularwashers 26, typically of mica and of larger diameter, stacked andarranged concentrically along an axial passage 27. With this arrangementa series of gaps exist between the inner margins of the washersextending across the inner margin of the intervening mica washer. Theaforesaid stack assembly may be disposed within a mounting housingincluding a metallic cylindrical body portion 28 and a conical nozzlecap portion 29 affixed thereto, e.g., by a threaded joint 31. Conicalcap portion 29 is provided with an orifice 32 axially aligned withchannel 27 of the washer assembly and is in direct electrical contactwith a washer 24 at one end of said assembly. Support for the oppositeend of said washer assembly is provided by means of a tubular conductor33, retained concentrically within the cylindrical body portion 28, byan annular insulating member 34 fitted into a stepped portion of theinterior wall of body portion 28. The interior end of conductor 33 abutsin direct contact with a washer 24 at the second end of the stackedwasher assembly while the second end of conductor 33 projects beyond thefree end of cylindrical body 28. An insulating sealed cap 36 affixed tothe free end of body 28 may be used to support the conductor 33 thereat.

The metallic washers 24 comprise a metal such as titanium or zirconiumwhich have been treated to occlude a large amount of a gas such ashydrogen, deuterium, tritium, etc., from which it is desired to producea suitable ion beam. More specifically, the titanium washers may beheated in high vacuum to outgas impurities therein and then contacted,e. g., with H,, D etc., at an elevated temperature to initiate ahydriding reaction therewith. Details of such a preparation aredisclosed, for example, in Report No. UCRL-4496, issued by theUniversity of California Radiation Laboratory, abstracted in NuclearScientific Abstracts, Aug. 3 l, 1955.

The plasma source 23 also includes a trigger electrode 37 disposed ininsulated sealed relation within conductor 33 having an inner enddisposed in proximity to the inner end of conductor 33 and the washer 24abutting therewith and defining a spark gap 38 therebetween. A pulsepower source is connected across the stacked washer assembly to furnishpower for generating the plasma. Such a power source may comprise apulse line 41 comprising a series of capacitors connected to a tappedinductor and including damping and current limiting resistors as inconventional practice. A direct current power supply 42 is connected toone end of the pulse line while the other end of the pulse line isconnected between conductor 33 and the grounded mounted housing of theplasma source to establish a suitable potential across the stackedwasher assembly. A means for generating a spark discharge across sparkgap 38 is provided to initiate a discharge of pulse line 41 along theseries of gaps between the washers in axial passage 27. Such means maycomprise a pulse transformer 20 having the secondary connected betweenconductor 33 and electrode 37. A trigger pulse applied from a triggergenerator (not shown) to the primary of said pulse transformer will thenproduce a spark discharge across gap 38 which discharge will initiatedischarge of the pulse line energy across the washer gaps as notedabove. Such a discharge causes gas to be released from the metallicwashers 24 to be ionized and heated and be ejected from passage 27through orifice 32 axially along housing section 13, to form plasma body43 therein. The plasma of body 43, with a hydride source will generallycomprise H' ions, electrons and a substantial proportion, e.g., 50percent of Ti ions. Flow of pulse current from pulse line 4] can beterminated, for example, by short circuiting the output therefrom as bymeans of a triggered ignitron 44 or other switch connected across theoutput of the pulse line. A substantially square wave pulse of selectedduration can then be applied to such source. Voltages in the range of afew KV to at least 20 KV can be employed with such a pulse line.

For purpose of extracting ions from plasma body 43 a mosaic element 46is disposed to intercept or otherwise be disposed to be in closedproximity to plasma body 43. The matrix element 46 may be formed of anymaterial suitable for vacuum service and it may be either conductive ornon-conductive. Suitable materials include glass, ceramic, metals suchas stainless steel, nickel, etc. The element may take the fonn of aplanar disk plate, as shown, or it may be a curved section correspondingto a cylindrical or spherical plasma body, or any other suitable shape.The element must be provided with a large plurality of very smallchannels, preferably, substantially parallel straight line channelstranspiercing the mosaic element. The element is arranged preferablywith such channels aligned with the path of the ions leaving plasma body43.

In the event that magnetic field, supplied, e.g., by solenoids woundabout vessel section 13, is used to combine or guide the plasma, thechannels must also be aligned with the magnetic field line guidingcenters for effective extraction to occur. Generally speaking thechannel diameter must be below about 50 microns and usually below about25-30 microns dependent, as indicated above, on the Debye length of theparticular plasma. With particle densities of the order of particles/cc,channel diameters of the order of 10-15 microns or smaller yieldsatisfactory or superior results and are preferred. Such elementextracts ions from the plasma body 43 to form a beam 45, e.g., of H ionsemergent from the opposite side to enter housing 17.

One highly satisfactory mosaic element formed of a material,commercially available, comprises a glass Pyrex) element which onmicroscopic examination can be seen to be a polycellular or honeycombmosaic in which the capillary channel walls have a hexagonal thin wallconfiguration leaving at least about 50 percent open channel space.Disks of such material having channel sizes such as l0 and 25 micronsdiameter are available. Versions having a metallic plating on one orboth sides are also available and are suitable. Similarly suitablemetallic polycellular mosaic materials can also be made as disclosed inUS Pat. No. 3,222,144 issued Dec. 7, 1965 to Donald E. Davenport. A thinmatrix element is generally preferred since the shorter the channellength the higher the ion beam transmissivity, due to improved geometry,yielding larger ion currents. The current output can also be increasedby increasing the ratio of open channel area present, by increasing theincident particle energy and by reducing the radial energy of theparticles, i.e., the ratio of rotational energy to translational energy.The probability (P0) of passage through the channel of a particle at thecenter of the channel is He) A (Vz/Vr) 2 (R/L) where Vz translationalvelocity along capillary axis, Vr= velocity transverse to axis, L=length of channel and R radius of channel. Matrix elements of the orderof 0.025 inch thick for a 25 micron hole size and 0.010 inch thick for a10 micron hole size or less are satisfactory.

The mosaic element 46 may be mounted, for example, by means of acup-shaped alumina mounting holder 47 secured to the base plate 16,transverse to the axis of vessel 13 within the aperture defined by theannular base plate 16. The bottom 49 of means 47 is apertured centrallyand mosaic element is disposed to cover said aperture in bottom 49 ofmeans 47. The mosaic element 46 may be retained in position by afunnelshaped guide element 51 secured to plate 16 by a mounting ring 52,or by any other suitable retaining means. The walls of element 51coverage on an axial aperture 53 therein which defines an area of mosaic46 toward which the plasma body approaches or is in proximity to.

In usual applications, evacuation of chamber 12 may be provided bycommunication through the channels of the mosaic element to the regionwithin vacuum housing 17 evacuated with the vacuum pump means (notshown) as customarily utilized. However, for other purposes, e.g., whenmaking measurements of performance, etc., the housing 17 can be a vacuumtank evacuated to below 10' to 10" mm Hg or lower by a vacuum pump (notshown). In such a case current deposited on the mosaic element 46, guideelement 51 or grid 54 may be measured by a current detecting loop i.e.,a bug) 56 disposed about a ground lead 57 connected to plate 16.Collector grids, double probe detector means or the like may be providedin housing 17 to measure ion currents and other effects as inconventional practice. However when utilized as an ion source, e.g., inan accelerator, etc., plate 16 may merely be grounded.

Typical constructional details and operating parameters for such an ionsource are set forth in the following illustrative example:

EXAMPLE Vessel section 13, 2 inches I.D. Pyrex glass tubing, 6 incheslong Source 10 hydrided titanium washers A inch O.D. 3/32 inches l.D.ca. 1 mm thick Pulse line: six 8.5 microfarad capacitors distributedalong a 210 micro henry induction coil Typical pulse lengths 5-l 75microseconds Charging voltage across pulse line 2 KV to 15 KV Arcvoltage drop across washer stack ca. v. Evacuation pressure down to5X10" mm Hg.

Source to mosaic spacing ca. 8 cm Mosaic glass hexagonal channel 25micron hole diameter 0.025 inch thick 10 micron hole diameter 0.010 inchthick (Gold plated bothsides and unplated) Current density at leastabout 0.10 to 1.0 amp/cm Plasma ion energy mean ca 1 ev Ion energy (I-I)in ion beam about 50-150 ev Results indicated that at 5 KV charge,productive of a higher density plasma, more current passed through the10 micron mosaic than through the 25 micron mosaic. This is because theDebye length of such a plasma is about 25 microns and the 25 micronmosaic tends to defocus the plasma surface. With a 10 micron mosaic theplasma sheath forms much more evenly. With a 3 KV charge, the density isless and the Debye length longer so that defocussing no longer occurs.Moreover, in the first mentioned case the electron current is about fivetimes as large as the ion current using the 25 micron mosaic. With the10 micron mosaic, ion and electron currents are about equivalent.

DESCRIPTION OF A SECOND EMBODIMENT A second embodiment 75 of an ionsource in accordance with the invention as illustrated in FIG. 2 isconstructed with a tubular microwave permeable vacuum vessel 76constructed, e.g., of ceramic or glass defining a chamber 77 therein. Amosaic element 49, of the character described is disposed across theopen end of such vessel 76 together with a grid 49, as above. A conduit79 may be used to supply a gas, e.g., hydrogen, deuterium, tritium,helium, etc., at the desired density to chamber 77. The vessel 76 may bedisposed within a waveguide 81 coupled to a microwave energy source (notshown) for application of microwave energy of a suitable frequency andat an appropriate level to ionize and heat the gas in chamber 77 toproduce a plasma body 82 therein. To regulate the plasma potential anelectrode 83, e.g., of tungsten, etc., may be passed in sealed relationthrough the closed end 84 of vessel 76 with the inner end 86 thereofdisposed in plasma body 82. A positive potential, applied from a DC.power supply (not shown) applied to electrode 83 will regulate theplasma potential and thereby control the energy of ions emerging throughmosaic 46 in ion beam 87.

Certain relationships may be used to determine the operating parametersfor such an ion source as follows:

The plasma frequency f,,,,,,,,,,, =fy)t= 9 l0 ne frequency of microwavepower to be applied. (Ne plasma density particles/cc.) For exampie, withne 10" particles7czfnk= 9X10 and k 3 cm microwaves may be varied over aconsiderable range, e.g., up to at least to several hundred volts givingequivalent ion energies.

While there have been described in the foregoing what may be consideredto a preferred embodiments of the invention, modifications may be madetherein without departing from the teachings and scope of the inventionand it is intended to cover all such as fall within the scope of theappended claims.

What I claim is:

10 1. An ion source comprising:

plasma source means including a stack of interleaved metalfunnel guideelement means including walls converging toward a central aperture whichis axially aligned with said channel and arranged to collect and directsaid plasma stream toward said central aperture; and

means including a substantially planar mosaic disk element disposedtransversely across said aperture, said mosaic element defining aplurality of straight line channels substantially in alignment with theaxil flow vector of said plasma stream through said aperture with thediameter of 0 said channels being less then the Debye length of saidplasma stream so that a plasma sheath potential is developed in theplasma in proximity to said mosaic element whereby electrons therein aredecelerated and ions are accelerated therefrom at an energy above theirinitial kinetic energy in the plasma stream to pass through the channelsin said mosaic element to form an energetic ion beam emergent therefrom.

2. Apparatus as defined in claim 1 wherein said plasma has a density ofat least about 10" particles/cc providing a Debye length below about 25microns and wherein said channels have a diameter less than about 25microns in magnitude.

3. Apparatus as defined in claim 2 wherein said plasma source meansfurther includes a cylindrical housing disposed in insulated relationabout said stack of interleaved washers said housing having a conicalcup portion defining an orifice axially aligned with said channelthrough which said plasma stream is directed and wherein said metallicwashers are formed of a material selected from the group consisting oftitanium and zirconium.

4. Apparatus as defined in claim 3 further including, a vacuum housingdefining an evacuable chamber said housing including a central sectionto one end of which is attached to cover plate centrally apertured toreceive and support said plasma source housing and to the opposite endof which is attached a base plate centrally apertured to receive andsupport said guide element means with the aperture in axial alignmentwith the washer stack channel as well as to support said means includingsaid mosaic element with the channels thereof in axial alignment withthe aperture of the guide element.

5. Apparatus as defined in claim 4 wherein said mosaic element comprisesa disk of glass material, said glass material being disposed in ahoneycomb configuration with thin wall portions defining said pluralityof straight line channels, said straight line channels being insubstantially parallel alignment.

6. Apparatus as defined in claim 4 wherein a charging voltage in therange of about 2 kilovolts to about 15 kilovolts is applied across saidwasher stack by said means for creating a discharge between said washermembers, wherefor the plasma ions have an energy in the range of about50 to electron volts, wherein the channels of said mosaic elements havea diameter in the range of about 25 microns to about 10 microns and athickness in the range of about 0.025 inches to about 0.010 inches.

7. Apparatus as defined in claim 6 wherein said mosaic disk element isprovided with a metallic plating or at least one side thereof.

* i F k

1. An ion source comprising: plasma source means including a stack ofinterleaved metallic annular and insulating material washers defining anaxial channel therein, said metallic washers having a material selectedfrom the group consisting of hydrogen, deuterium and tritium occludedthereon, and means for creating an electrical discharge between portionsof said washer members within said channel so as to release, ionize andheat said occluded gas to generate a heated plasma stream containingelectrons and ions which is ejected axially from said channel; funnelguide element means including walls converging toward a central aperturewhich is axially aligned with said channel and arranged to collect anddirect said plasma stream toward said central aperture; and meansincluding a substantially planar mosaic disk element disposedtransversely across said aperture, said mosaic element defining aplurality of straight line channels substantially in alignment with theaxil flow vector of said plasma stream through said aperture with thediameter of said channels being less then the Debye length of saidplasma stream so that a plasma sheath potential is developed in theplasma in proximity to said mosaic element whereby electrons therein aredecelerated and ions are accelerated therefrom at an energy above theirinitial kinetic energy in the plasma stream to pass through the channelsin said mosaic element to form an energetic ion beam emergent therefrom.2. Apparatus as defined in claim 1 wherein said plasma has a density ofat least about 1012 particles/cc providing a Debye length below about 25microns and wherein said channels have a diameter less than about 25microns in magnitude.
 3. Apparatus as defined in claim 2 wherein saidplasma source means further includes a cylindrical housing disposed ininsulated relation about said stack of interleaved washers said housinghaving a conical cup portion defining an orifice axially aligned withsaid channel through which said plasma stream is directed and whereinsaid metallic washers are formed of a material selected from the groupconsisting of titanium and zirconium.
 4. Apparatus as defined in claim 3further including a vacuum housing defining an evacuable chamber saidhousing including a central section to one end of which is attached tocover plate centrally apertured to receive and support said plasmasource housing and to the opposite end of which is attached a base platecentrally apertured to receive and support said guide element means withthe aperture in axial alignment with the washer stack channel as well asto support said means including said mosaic element with the channelsthereof in axial alignment with the aperture of the guide element. 5.Apparatus as defined in claim 4 wherein said mosaic element comprises adisk of glass material, said glass material being disposed in ahoneycomb configuration with thin wall portions defining said pluralityof straight line channels, said straight line channels being insubstantially parallel alignment.
 6. Apparatus as defined in claim 4wherein a charging voltage in the range of about 2 kilovolts to about 15kilovolts is applied across said washer stack by said means for creatinga discharge between said washer members, wherefor the plasma ions havean energy in the range of about 50 to 150 electron volts, wherein thechannels of said mosaic elements have a diameter in the range of about25 microns to about 10 microns and a thickness in the range of about0.025 inches to about 0.010 inches.
 7. Apparatus as defined in claim 6wherein said mosaic disk element is provided with a metallic plating orat least one side thereof.